1. Field of the Invention
The present invention relates to spectroscopy and in particular to a near-field heterodyne spectroscopy that facilitates small spot measurement while minimizing probing depth and eliminating background signal in the detection field.
2. Description of the Related Art
Semiconductor metrology, such as monitoring thin film thickness, microstructures, carrier concentration, and strain/stress, is based on electromagnetic wave interaction and its electric field effects on the dielectric function of the material. As the size of the semiconductor device feature shrinks to submicron scale, contactless, non-invasive metrology tools that rely on electromagnetic wave spectrum (spanning from deep ultraviolet (DUV) to far-infrared (THz wave)) to determine material properties face challenges in meeting the spatial resolution imposed by the feature size while still offering sufficient sensitivity.
Photo-reflectance modulation spectroscopy uses the dielectric response of a material from electromagnetic waves to measure specific properties of the material. The spectral response of the material can be modulated by applying an external repetitive perturbation of electromagnetic waves (i.e. a pump beam). The response to the electromagnetic waves can be detected by a reflected probe beam. Specifically, differential changes in the reflectivity can be related to the perturbation of the complex dielectric function ∈=∈1+i∈2 (wherein ∈1 is the real part of permittivity and ∈2 is the imaginary part of permittivity) via the following relation:
                    Δ        ⁢                                  ⁢                  R          ⁡                      (            λ            )                                      R        ⁡                  (          λ          )                      =                  α        ⁢                                  ⁢        Δ        ⁢                                  ⁢                              ɛ            1                    ⁡                      (            λ            )                              +              β        ⁢                                  ⁢        Δ        ⁢                                  ⁢                              ɛ            2                    ⁡                      (            λ            )                                ,          ⁢                    Δ        ⁢                                  ⁢        R            R        =                  10                  -          4                    ∼              10                  -          6                    and is capable of detecting
wherein R(λ) is the spectral reflectivity, ΔR(λ) is the spectral change of that reflectivity resulting from frequency modulation, and α and β are the Seraphin coefficients. The modulation frequency and wavelength of the pump beam could be optimized for specific applications, e.g. determining material properties, thickness, strain in the thin film, and dopant concentration. However, of severe limitation of the current photo-reflectance modulation techniques is the spatial resolution, i.e. both the lateral and longitudinal dimensions of the sample to be probed by the electromagnetic waves.
Known spectroscopy techniques use a far-field mode, i.e. the distance (L) between sample and pump/probe beam is much larger than the wavelength (λ), i.e. L>>λ. In these techniques, both the pump beam and the probe beam are focused by conventional diffraction limited optical elements, e.g. lens, beam splitters, and mirrors.
Unfortunately, in a far-field mode, the spatial resolution is diffraction limited (in practice for metrology, the diffraction limit is much bigger than that given by the Rayleigh criterion because the tails of the response function significantly affect quantitative measurement results):
  d  =            1.22      ⁢                          ⁢      λ        NA  
where d is the lateral spatial resolution, λ is the wavelength, and NA (the numerical aperture) is equal to n sin θ, where θ is half of the angular aperture on the object side and n is the refractive index of the medium above the target. Additionally, when using a far-field mode, the penetration depth of the electromagnetic wave in the material is limited by the wavelength either of pump beam or probe beam used (whichever is the shortest). For a laser-based, photo-reflectance system, the penetration depth δ of light into a non-absorbing material is related to wavelength λ as follows:
      δ    p    =      2.53    ⁢          λ                        (          NA          )                2            
For an absorbing material, the penetration depth is related to the wavelength (λ) and extinction coefficient k(λ) of material as follows:
      δ    p    =      λ          2      ⁢                          ⁢      π      ⁢                          ⁢      k      
Thus, the minimum measurement site size is limited by the diffraction and penetration of the probe and pump beams.
Notably, because both the probe beam and pump beam are far-field in nature, the scattered luminescence signal from the non-monochromatic pump beam and any background luminescence will be inadvertently collected by the probe beam optics, thereby degrading the signal-to-noise (S/N) ratio.